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AF447 stall and failure to recover
To my knowledge Air France Flight 447 experienced stall. How was the stall detected and why wasn't the crew able to recover?

You can find what you want here

I gave a pretty detailed answer regarding AF447 and the stall detection/recover in this answer

The stall, for the most part, was not detected by the pilots: That was the entire problem.
There were many alarms and indications in the cockpit and that caused crew confusion, they neither knew which data to believe nor understood the situation that they were in. For much of the descent they could have recovered from the stall if they had been fully aware of the problem and had worked together. One pilot did try to recover the situation (by telling the other pilot to push the nose of the aircraft down) but he was not successful.
That answers the second part of your question too: The crew was able to recover from the stall but did not, due to their own confusion.

What is a Stall?
A stall means that the flow of air over the wing is insufficient to produce enough lift to keep the aircraft in the air. This can be related to airspeed and angle of attack. Increase the angle of attack enough and the flow will separate from the airfoil.
Why not recovered?
From the investigation report carried out by the BEA, the pilots both reacted differently to the stall warning (which was false, frozen pitot tube). The aircraft entered a stall, which was perfectly correctable, and started falling down.

The stall warning was absolutely true. Stall warning is generated by the AoA vane and that was not frozen.


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Flying an amphibian with ASEL privileges
So I have an interesting scenario:
You hold an ASEL private pilot certificate in the USA with high-performance and complex endorsements. You also fly a Cessna 206 on a regular basis.
Given that all your certs and currencies are valid, are you allowed to act as the sole PIC of a Cessna 206 amphibian as long as you perform your takeoffs and landings on land only?

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sort: votes


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What is the difference between SUA and SAA?
I would like to understand the difference between Special Use Airspace (SUA) and Special Activity Airspace (SAA). I found the following definitions.
Eurocontrol Lexicon for SUA (from FAA):

Airspace of defined dimensions identified by an area on the surface of
  the earth wherein activities must be confined because of their nature
  and/or wherein limitations may be imposed upon aircraft operations
  that are not a part of those activities.

Eurocontrol Lexicon for SAA (from FAA):

[U.S.] Any airspace with defined dimensions within the National
  Airspace System wherein limitations may be imposed upon aircraft
  operations. This airspace may be restricted areas, prohibited areas,
  military operations areas, air ATC assigned airspace, and any other
  designated airspace areas.

Can you please help me to understand what the difference is?

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Why do the Diamond DA-42 and DA-62 have a minimum fuel temperature requirement?
On the turbo diesel DA-42 and DA-62 aircraft there exists a minimum fuel temperature limitation where you may not start or operate the aircraft below that.  Does anybody know why this is?  It seems strange as the turbo Diesel engines themselves have their own dedicated engine fuel pumps which raise the pressure of the fuel to 19,500 psi prior to infection in the cylinders.  This should make the fuel plenty hot for combustion.
Is this minimum fuel temperature requirement typical for all reciprocating Diesel engines?

Look up diesel gelling to understand what happens to diesel at low temperatures.

I’m surprised at that considering the airplane runs on Jet A which I don’t believe can gel at that high of a temp.  I think the lower limit is something like 21° F

"which I don’t believe can gel at that high of a temp" So for the benefit of those of us who don't fly a DA-42 or DA-62, what is this minimum fuel temperature? Also, specifying in the question that the engines are supposed to run on Jet A might be beneficial.

The DA-62 AFM states that the operational fuel temperature is between -30° to 60° C

From the DA-42 TDI Approved Flight Manual:
Fuel filter is not heated and can clog if the Diesel fuel flocculates. Jet fuel does not pose same limitations.

Why is blending diesel and jet fuel in the tanks prohibited?  Wouldn't even 50% or so of jet fuel prevent filter clogging (although not as well as 100% jet fuel) by decreasing the freezing point of the mixture below that of pure diesel?

@Sean There are different limits based on which fuel or fuel blend is used, mixing isn’t prohibited but it still carries a limitation.

Jet fuel also has limitations but way lower.  Jet A1 and JP 8 are -37C for dispatch and have to stay above -47C in the tanks while flying.

Well that’s the thing.  I’ve never heard of a DA-42 or -62 operating on Diesel.  They only use JET-A or similar jet fuel.

@CarloFelicione That's because diesel isn't generally available at most airports, it would be more common in places like South America or China.

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Can an FAA Pilot Examiner accept BasicMed in place of a medical certificate from a checkride applicant?
This has been a subject of debate recently. §61.23 states that you can use BasicMed to apply for some certificates or ratings but limits this to gliders, practical examination in a simulator or flight training device and balloon categories. For other ratings, unless you were a former pilot with the armed forces and applying for a conversion of your military ratings, you must present at least a valid third class medical certificate to a pilot examiner when applying for a rating.
I talked with the FAA Medical Certification Division and they seemed to think one could apply for a airman certificate using only BasicMed.  does anyone out there know if Pilot Examiners in the field are accepting BasicMed with a valid driver’s license from checkride applicants?

I think you may be confusing Basic Med with No Medical At All, some operations do not require any sort of medical exam

(b)Operations not requiring a medical certificate. A person is not
  required to hold a medical certificate -
(1) When exercising the privileges of a student pilot certificate
  while seeking -
(i) A sport pilot certificate with glider or balloon privileges; or
(ii) A pilot certificate with a glider category rating or balloon
  class rating;
...
(8) When taking a practical test or a proficiency check for a
  certificate, rating, authorization or operating privilege conducted in
  a glider, balloon, flight simulator, or flight training device; or

Basic med can be accepted if you comply with the Basic Med regulations. However you need at least one valid medical (that can be expired) to be eligible for basic med, since a Class III medical is valid for 5 years (under the age of 40) its more than likely your training will be accomplished before it expires and thus you will have a valid medical when you sit for your check ride.
The FAA address this in this Q/A document

Q17: Do student pilots need to hold a medical certificate when they go
  for their private pilot checkride?
A: Individuals who are new to
  aviation will need to get one medical certificate if they want to fly
  under BasicMed. After that, they can fly under BasicMed forever, as
  long as they fly a BasicMed-compliant aircraft and never fly for
  compensation or hire. Most student pilots will still hold their first
  medical certificate when they apply for their private pilot
  certificate.

To trace the legislation exactly:
As you note for some student pilot operations as per 61.23

(c)Operations requiring either a medical certificate or U.S. driver's license.
...
(v) Exercising the privileges of a student,
  recreational or private pilot certificate if the flight is conducted
  under the conditions and limitations set forth in §61.113(i); or

This would potentially allow it but sends you over to 61.113(i) for more info

61.113 Private pilot privileges and limitations: Pilot in command.
...
(i) A private pilot may act as pilot in command of an aircraft without
  holding a medical certificate issued under part 67 of this chapter
  provided the pilot holds a valid U.S. driver's license, meets the
  requirements of §61.23(c)(3), and complies with this section and all
  of the following conditions and limitations:

which just bounces you right back to §61.23(c)(3) one section down from where we were earlier

...
(3) A person using a U.S. driver's license to meet the requirements of
  paragraph (c) while operating under the conditions and limitations of
  § 61.113(i) must meet the following requirements -
(i) The person must -
(A) Comply with all medical requirements or restrictions associated
  with his or her U.S. driver's license;
(B) At any point after July 14, 2006, have held a medical certificate
  issued under part 67 of this chapter;

So at some point you will need to get at least one medical. Most students do this along with either student pilot certificate (allowing them to fly solo) furthermore most complete their training within 5 years of that date although there are other cases. For example, you started your training in early 2006, got you class III medical/student pilot on July 15th of 2006 which is good for 5 years. Life gets in the way and you stop training for a while. Your medical expires in 2011 but you now decide to get back into the left seat. Since you never lost your medical and once held a valid medical you are legal to fly and train under basic med (and even sit for a check ride).

Yes, you can use BasicMed in place of a third class medical. When BasicMed was introduced, the FAA updated 61.23 (I've bolded the new text):

(3) Must hold at least a third-class medical certificate—
[...]
(iii) When taking a practical test in an aircraft for a recreational
  pilot, private pilot, commercial pilot, or airline transport pilot
  certificate, or for a flight instructor certificate, except when
  operating under the conditions and limitations set forth in
  §61.113(i); or

61.113(i) states the BasicMed limitations.
On the other hand, if you check the FAA's checkride guidance to examiners (e.g. for an ATP), it says:

At the time of application for an ATP practical test, the applicant must hold at least a current third-class medical certificate.

So, the regulations say BasicMed is fine, but the examiner guidance says it isn't; the obvious explanation is simply that the FAA hasn't updated every document affected by BasicMed. The regulation itself is clear and takes precedence over a guidance document anyway.
If a DPE did question using BasicMed for a checkride instead of a third-class medical, I think an appropriate response would be to show them the current version of 61.23 and ask them to clarify.

BasicMed doesn't apply to ATP, so there seems nothing wrong with that guidance for DPE's

@TomMcW True, but this question is about which medical you need to take a checkride, not which one you need to exercise privileges. "Which class of medical is required for a checkride?" is a classic written and oral test question. With BasicMed, the answer got a bit more complicated.

I guess if you've always been allowed to take an ATP checkride with a third class then you should be able to use BM. Not sure why you'd want to since you couldn't actually use the ATP with them.

@TomMcW Some folks add ratings they never intend to use for novelty, to lower their insurance rate or to avoid a BFR.

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What spacing is needed between aircraft parking stands?
I'm doing a project where I have to design an apron for a general aviation airport (ICAO type 2B). The airplanes have to park in angled nose-in parking, or angled nose-out parking.
What is the required separation between two stands? I think that the dimensions of one stand depend on the turning radius of the airplane that is supposed to use the stand, but I don't know where to find this angle for every airplane...
I'm looking for any data or other relevant information.

I know I've parked 172s where there was less than 12" from wing-tip to wing-tip of adjacent planes.   It took two pushers and two wing-walkers to put planes in safely.

I think most GA aircraft can lock the brake on one wheel and pivot around it.  Also, when I park I almost always shut down the engine and push the plane into the spot.  See also this list of airplane sizes

Sure, let's have some spacing now!

The Aerodrome Standards manual, which is based on ICAO Annex 14, gives the following values:

Clearance distances on aircraft stands
3.12.6 An aircraft stand shall provide the following minimum clearances between an aircraft using the stand and any adjacent building, aircraft on another stand and other objects:
Code Letter | Clearance
A | 3 m
B | 3 m
C | 4.5 m
D | 7.5 m
E | 7.5 m
F | 7.5 m

A references for the ICAO code letter categories is available here: http://www.skybrary.aero/index.php/ICAO_Aerodrome_Reference_Code

Yes, in this case the distance between two planes is 3 meters. But the distance between two stands isn't 3m+wingspan+3m, because the plane has to park with its own propulsion, so has to do a 180º turn in this space.

To be fair, you did ask for the width between stands, not the width of individual stands

@oriolflvi While J. Hougaard's comment might appear to be nitpicking, on Stack Exchange, we generally do aim to answer the exact question that was asked. If you realize before getting any answers that what you asked wasn't quite what you meant to ask, it's common to edit the question to fix that. As this stands, with an upvoted answer, I would encourage you to simply ask a new question, and in it, be more specific about what exactly you are looking for. There's no shame in linking to this and saying "I asked this other question too, but it wasn't exactly what I meant because XYZ".

Even us old-timers on the network get it wrong on occasion, realizing that we asked a perfectly valid question, got perfectly valid answers to it, but that it turned out that the question we asked wasn't what we wanted to ask. In that case, it's often best to simply accept that what you asked wasn't what you meant to ask, treat the question as asking what it actually asked instead of what you meant to ask (so vote, accept etc. based on that), and ask a new, better question drawing from the experience thus gained, highlighting the differences between the two questions (to avoid dupe votes).

Thank you! I'm going to explain better the problem and what i'm looking for. I never saw this forum and I've seen that it's very interesting and a useful tool, so thank you for explain me better how it works.


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What is this unit above the autopilot panel in Boeing test aircraft?
I keep seeing this (more here and here) unit (maybe solely) in Boeing test airplane cockpits above the autopilot panel. Can someone tell me what is this and what the purpose of this big unit? Based on this video the center display tells you the time (UTC?) but cannot figure out the purpose of the 2 side displays. Not to mention having a clock in the cockpit does not require a huge unit like this I assume even in aircraft safety terms with being spark proof...).
Also would be interesting to know what is that small black panel to the right of the bottom FMC here. Looks some buttons have arrows, maybe camera control panel?

This is not an integrated unit. It was likely assembled by Boeing. The center display is an airborne IRIG precision time display made by the Datum Corporation, (look at the logo bottom left of the display) which merged with Symmetricom in 2002 so references are scarce. Here is a similar but different model which has a brief description of capability.
IRIG is used to communicate timecodes from precision references such as atomic clocks, in this case as transmitted by GPS satellites. The devices on either side are not marked, but they would be used by each test pilot to record timestamps as they begin and complete specific actions in their test plan. 'Address' on the devices would be used to specify the location of each timestamp and likely autoincrements after each use.

This is an educated guess. It is my assumption that it is a quick-access flight data recorder, cockpit voice recorder, or both. I believe this because of the two units that flank the clock. As @ocirocir stated in their comment, the extremely precise clock is for timing of events. Test pilots and flight engineers keep close documentation upon what happens during flight. With today's technology, it would stand to reason that some of this documentation could and should be done digitally, either as primary record or to supplement and cross-verify with the test flight crew.


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How does Integrated Modular Avionics (IMA) improve reliability and obsolescence mitigation over the federated architecture?
I've been researching this question on why IMA is more reliable, and the common answer seems to be because of the robust partitioning which is reliable and can prevent software fault propagation.
But how is it more reliable? Wouldn't sharing of hardware resources create more dependencies for these systems?
How does IMA prevent obsolescence comparing it to federated architecture?

Software faults are not included in reliability calculations because a software fault is a design issue. Developing SW in accordance with DO-178C (or predecessors) is intended to ensure that these faults are minimized. The reasoning that it's not measured as reliability is that when there is a SW fault it's not a broken box on the plane that has to be fixed, that fault is a potential failure on every aircraft that has the same system.
Swapping out a unit for a SW fault doesn't solve the problem. It's going to take a service bulletin (SB) with new SW deployed to the fleet to fix it.
So reliability is about hardware failures. Basically, parts break. Some break more often than others. The more parts you have, the more likely something will break. So making things more reliable is about minimizing parts and using the most reliable parts you can. But there are limits to what you can do. A power transistor will be less reliable than a resistor, but if you need it, you have to have it. But having one or two instead of 3 or 4 will improve reliability. I suggest a read of MIL-HDBK-217 which is the industry standard for predicting reliability.
So an IMA improves reliability by reducing the number of Line Replaceable Units (LRUs). Fewer LRUs means fewer cables, connectors, power supplies, and processors. An IMA can run multiple functions on a single processor card instead of having a dedicated unit for each function. All that means fewer parts to fail and higher reliability.
Aside from parts count, the biggest drivers of electronics failures are heat (high temps and high temperature swings) and vibration. Which is why some manufacturers perform Highly Accelerated Life Test (HALT) to identify weak points in the design early.
Nothing will solve the issue of parts obsolescence. Parts will go obsolete. It's a huge problem in aviation where products hang around for 20 or more years. An IMA does help manage the obsolescence issue. It does it by having a bunch of common processor boards. Not only do IMAs have fewer total parts, they have fewer unique parts which mean fewer parts that can go obsolete. Fewer parts going obsolete will mean less work to replace them.
It's still a complicated process to deal with, but if a part becomes non-procurable on an IMA processor board you'll need to modify the board design to use a new part. That can be a fairly straight-forward part replacement (equivalent part) to a major redesign of the circuit board. In the end, the new board has to be equivalent to the old one. That way it can be swapped out without impacting anything else (software or hardware).

I remember the big scramble with the change from Mil-Spec to Mil-Perf, and the ending of production of Mil-Spec compliant ICs and microprocessors in the 90s.  The black box makers had to scramble to adopt commercial hardware by filtering component batches with Mil-Perf screening testing to get the required quality.


text:
Definition of aircraft longitudinal axis?

I know that longitudinal  axis connect nose and tail, but at what exactly point?
What definition says?
I have a glider and I want to draw with pen longitudinal axis to measure angle of incidence  for wing and stabilizer, but I don't know where exactly I need to draw longitudinal axis.

Is airflow parallel with longitudinal axis during cruise, for well design real aircraft(to reduce fuselage pressure drag)?

Principally, the axis is set by the manufacturer. But there are some choices which are preferable over others. For flight mechanics, it is good practice to use the main inertial axes for the coordinate axes.

That sounds like an answer--

@quietflyer … but is too short. Since I have enough reputation, I leave it to others. Why did I write it then? I was not satisfied with the existing answer

Aircraft designers use many different axis frames, depending on what is the most convenient under the circumstances.
In the context of the question, I need to mention only two of them, or rather even - two classes of them: body axes and wind axes.
The body frame is tied, well, to the body. It is rigidly fixed to the body and moves with it. By convention, the X axis points "forward", to the nose.
The wind frame is aligned with the streamflow "at the infinity", and its X axis points against the free stream. If you rotate the wind frame by the wind angles - angle of sideslip and angle of attack - you get to the body frame.
The term "longitudinal axis" refers specifically to the body X axis.
But where is the origin? It depends!
For most calculations, especially in flight mechanics, it is best to put the origin at the centre of mass. Furthermore, the direction of the longitudinal axis, over which the designer has some liberty (that arbitrary "forward"), should be such that it formed (one of) the principal axis of rotation.
This is not always easy, nor necessarily convenient. Centre of mass moves, the aircraft may not be symmetric, and the airframe designer will not like the reference axes moving about.
To avoid ambiguity, it is often necessary to state in which system the calculation is done, and how exactly it is defined. Oftentimes, engineers in a certain sub-field (e.g. flight mechanics) assume a certain frame and understand each other well, but this assumed knowledge is not carried to other fields, leading to errors and misunderstandings.
The situation is exacerbated by the fact that in many practical cases, the difference between the frames seems to be small, and the result may be "approximately correct" in most of them.1
So, as a designer, you decide how you define the longitudinal axis. If the aircraft is relatively symmetric, draw it as the axis of symmetry.
Your choice will affect how you read angle of attack (AoA). There is nothing strange about that: AoA of the aircraft is also "conventional": it is with respect to an essentially arbitrary axis. It will likely be different to the AoA of the wing, which is of the most concern for the aerodynamicist. (One of the conventions in pure aerodynamics is to choose the X direction such that it resulted in zero lift at zero AoA).
Whether the airflow will align with the axis at cruise depends on your choice. You will need to choose the angle of incidence of the wing to minimise drag, but the AoA may not be zero if you chose the axis direction with other considerations in mind.
Note that the choice of origin will not affect the wind angles. Essentially, you can leave the choice of origin until you need to decide on the other two axes.

1 A common example: Lift is defined in the wind axes. It is normally linearly dependent (~proportional) on AoA, but this only holds perpendicular to wind, and not with respect to "up" of the aircraft. But the difference is small for small angles.

The longitudinal axis of an airframe on the drawing board is distinct from the longitudinal axis of the plane in flight.
On the drawing, it is an arbitrary reference line drawn by the designer. Typically it connects the most forward part of the nose with the most rearward part of the fuselage tailcone. Any angle of incidence of the wing and/or tailplane is referenced to this line.
In flight, it passes through the centre of mass but remains parallel to the design reference axis. It may or may not align with the forward direction in normal flight.
The axis in flight may be close to the line drawn by the designer, or it may not. There is often some displacement of the designer's axis from the center of mass, for example if the engine is mounted above a high wing as with many flying boats.
Usually the designer tries to get the direction as close as possible in order to minimise fuselage drag, which answers your second question.

This "aligns with the forward direction in normal flight" is not quite correct. It would be for the wind or trajectory axis frames, but these "X" axes are not actually called "longitudinal". Longitudinal is necessarily a body axis, and this means its direction does not necessarily align with the direction of flight.

Thanks, I have updated my answer accordingly. The relation to the CG position on the design drawings vs. in flight is still a significant one, especially for pitch trim.

Per the Pilot's Handbook of Aeronautical Knowledge: Longitudinal  axis.  An  imaginary  line  through  an  aircraft  from nose to tail, passing through its center of gravity. The longitudinal axis is also called the roll axis of the aircraft. Movement  of  the  ailerons  rotates  an  airplane  about  its  longitudinal axis.
PHAK Glossary.

The aircraft's coordinate axes originate from its centre of mass (gravity).
The longitudinal axis is not necessarily parallel to airflow, as its angle with airflow is angle of attack (AoA).
The roll axis is perpendicular to the other two axes with its origin at the center of gravity, and is directed towards the nose of the aircraft. A rolling motion is an up and down movement of the wing tips of the aircraft.

Source


text:
Does an RMI show true or magnetic north?
Does the compass card of a Radio Magnetic Indicator (RMI) point to true north or magnetic north ?

RMI (radio magnetic indicator) displays a magnetic direction, like the compass and a lot of things in aviation The RMI magnetic bearing leads to the navaid which is usually what the pilot wants. Navigating using magnetic north is also done by a walker with a compass. The magnetic declination describes how magnetic north and true north differ.

I removed your second question about using true vs magnetic because it looks like it's been answered here. If that answer doesn't help you, then you can just edit this question again, or ask a new question.

In a typical installation, the compass card of an RMI indicates magnetic north.
While it is possible to switch that reference on an EFIS display to true north by applying the local variation, that is not the common practice. If you see a "steam gauge" RMI (i.e. not a digital display, but a mechanical compass card) in a panel, chances are excellent that the compass card is slaved to the same heading source as everything else -- magnetic north. In the exception cases, that source can be switched between true & magnetic, but that's uncommon.


text:
Can radars detect small drones?
I've just started working with weather radars and such. With weather radars, it is possible to detect bird migrations with a specific algorithm and a lot of information about birds, but it would be a problem to detect a single bird (depending on the bird size) because of the radar's resolution.
I am wondering, if you have a place or a secret base that you don't really want the population to see whats in that area and take photos of the inside of the area or spy, is there a way to detect small drones with radar signals if you pack a few radars around your place or military base? If so, how?

For what I know, this could better fit in Physics.SE

From what I know of radars, it all depends on the frequency of the radar. High frequency = small wavelength = higher resolution = you can see drones and other small things if your radar has a high enough frequency (Which also means you need more power to run it).

Drone can be seen, depending on many conditions. Some aircraft are designed to be nearly invisible to radar, so the same can be done to drones, and that would work even closer to the radar system. This already exists.

I won't post an answer, since I hope that we have a radar expert here, but from my knowledge, this would highly depend not only on physics, but on the settings of the actual radar system itself. For example, it can happen that a radar designed to look for something else notices the drone, classifies it as clutter and doesn't show it at all. Also, again depending on system type, a radar might have issues with targets with close range, because it takes a bit of time to switch from transmit to receive.  There are commercial radars that can detect crawling infantry, for example FLIR Ranger R2.

@AndrejaKo: Good point on the radar software settings, but a FLIR uses body heat for detection, so it does not qualify in this context.

@Peter Kämpf  No, I meant FLIR as a company, not as a technology type. The company makes radars (well at least markets under its brand) as well as cameras and they have some systems that can even integrate both into "complete solution".

@AndrejaKo; You're right, I should have googled that before posting. The R2 operates at almost 40 GHz where the atmosphere is relatively opaque, so its range is severely restricted. But it is a radar.

Planning to fly your quadcopter over Area 51, are we?

I wonder if it might be easier to jam a drone's control signals

@CarloFelicione This one?

I might be wrong, but somehow I think it's probably easier to detect the signals being sent to and from the drone rather than trying to bounce stuff off it. Unless it's completely autonomous of course, in which case it appears your base wasn't secret after all...

Yes, small drones can be detected by radar, if the radar is designed for it. Aveillant  is manufacturing radars that have a high resolution and continously cover a volume of airspace in which it detects and tracks objects including drones. Their system was tested in Monaco earlier this year and they were able to detect small drones up to 5 km away.
Later this year they will install a permanent radar setup.

The Gamekeeper radar is designed to detect targets down to 0.01m2 cross section and has a range of up to 5km. Holographic Radar technology does not scan across an area as a traditional radar does, but continuously floodlights a volume of space, gathering 3D position and motion information from all targets, all of the time. This gives a detection and tracking capability beyond that possible with other radars.

Source
The technology they use is different from typical primary radars used for ATC.

Depends entirely on the drones. If the radar system is known and the drone is designed not to be detected by this particular radar system, then the drone can only be detected in exceptional circumstances. Small size would be one way, but in order to be effective, big is better.
Detection has many aspects, and one of them is how soon detection happens. If all you need is to stay undetected while you approach the target, only the frontal aspect needs to be considered. If, however, you want to stay undetected for the whole mission, the task becomes much, much harder.
The radar systems in operation during the Eighties along the Iron Curtain could already detect single, large birds. Adding computerized signal processing and networking of several radar stations has improved detection performance many times over since then. See this question for more details.

Radar detection of rotorcraft may be done by looking at the Doppler returns and seeing double sidebands caused by advancing and retreating rotors. The same is true for quadrotor type drones, though the radar returns from small plastic rotors may be very small.

You may improve your answer by speaking about radar resolution, as mention in the question.


text:
How can jet engines be more efficient than propellers?
If jet engines are smaller in diameter than propellers, is it fair to say that propellers accelerate a lot of air a little while a jet engine accelerates a little air a lot? Basically, a jet engine accelerates the air flowing through it by more than a propeller engine of the same thrust, right?
If so, propellers should be way more efficient because, by accelerating a larger mass of air, they're able to generate more force (thrust) while performing less work (change in kinetic energy of the accelerated air) due to the fact that force is proportional to $\Delta V$ but energy is proportional to $\Delta V^2$, but both are proportional to mass.
So, if this is true, shouldn't propulsion systems try to maximize the mass of air that it accelerates in order to minimize the $\Delta V$ required? This is, after all, the whole concept behind using large wings to stay aloft (they can grab onto a large mass of air).
So how can jet engines be more efficient than propeller engines? What other factors contribute to the efficiency of the propulsion system besides just the mass and velocity of accelerated air? And similarly, why aren't propellers way larger than they typically are? Because by this logic airplanes should have propellers the size of helicopter rotors.

Propellers become very inefficient when they cross the sound barrier, this limits tip speed and size...

Makes sense. So to propell an airliner to mach .85 its propellers would have to spin at supersonic speeds?

Depending on how big the props are, they can exceed mach 1 with the aircraft standing still on the ground...

This has got to have a dupe somewhere...  there is a huge amount of information out there about the relative efficiencies of props and jets in different flight regimes.  Please search around and come back if there’s something unclear in the other questions.

For the most part, yes. The most satisfactory explaination for me was the following: "Another factor is when you fly as fast as the propeller's tip speed—say Mach 0.75 (a Boeing 737 Classic in cruise)—the airfoil of each blade will be flying much faster than your forward speed (compare the hypotenuse to the vertical/horizontal sides in the topmost image)."

Propellors aren't way larger than they typically are for one fairly obvious reason: the bigger you make the propellor, the longer your landing gear needs to be.  Either that, or you have to make a tiltrotor: https://en.wikipedia.org/wiki/Bell_Boeing_V-22_Osprey

Why aren't propellers way larger than they typically are?

There are some "propellers" these days with a 400 foot disk diameter, known as windmills.
But, they are doing their best to extract energy from the airstream and here lies the issue.
"Relative wind" is conveniently used to combine the net direction and force of two aerodynamic effects, but let's look at an old question of:  can a sail boat ever sail faster than than wind?  This is very easy to explain by separating the "wind" components, rather than combining them.  Ice boats can go several times faster than the wind by using wind across the beam as "thrust", and using "wind" from the motion of the boat to create lift.
This is exactly how powered aircraft fly, the ice boat is simply doing it on its side (hence the tiller is the "elevator").
Which brings us to propellers, which can be more accurately described as generating lift than "moving air".  But, as the graph shows, net thrust of props decreases as speed increases because disk drag increases with speed.
Turbojets become more efficient at higher speeds than props because they have much less disk drag subtracting from their thrust.  Even though they are less efficient at producing thrust, their net thrust out put is much more linear over speed as "ram effect" (increasing amount of air available to produce more thrust) and drag cancel each other out, while the propeller falls victim to its own disk drag.


text:
What is oil thermal expansion in the CFM LEAP 1A engine for the A320NEO?
While I was going through the FCOM's SOP, normal procedures chapter, preliminary cockpit preparation section I came across the term oil thermal expansion. In the image below it is mentioned in the note section that

IF THE ENGINES HAVE BEEN SHUT DOWN FOR MORE THAN 60 MINS, DECREASE THE ABOVE QUANTITIES BY 3 QTS TO TAKE INTO ACCOUNT OIL THERMAL EXPANSION.

What exactly is this phenomenon and why does it require us to reduce the oil quantities mentioned when carrying out the engine oil quantity check?

As the oil heats up it expands and takes up more room. When the oil cools down it shrinks in size. This is a material property seen in almost all liquids, gasses and solids and called thermal expansion.
You have to fill in less when the engine is cool because once it heats up the oil will expand and all of the sudden you have too much oil.

@Jan then why isn't such a check mentioned on engines of different manufacturers. In my company we have IAEs as our CEOs and PWs on the NEOs. On both these engines the SOP doesn't call for such an adjustment. I have asked it also specifically in the question for the LEAP 1A.

Depending on engine make/model, this may or may not be a critical issue. Some engines just don't care about these things.

When oil heats up, its volume expands (and its density goes down). This means that if you measure the level of oil in a gearbox when it is hot, the level will read high, and if you come back later and measure it after it has cooled back down to ambient, it will read low.
So if you were to fill a sealed gearbox with cold oil without taking its thermal expansion into account, when it then gets hot it might expand enough to overpressurize the gearbox and blow out its seals.
I had a tractor engine in which overfilling it with cold oil would then cause it to pump the box containing its ignition points full of engine oil once the engine had gotten hot. This would then foul the points and stop the engine.


text:
How do I calculate the landing weight in this situation?
Empty weight 538kg
Pilot and passengers are 150kg with full fuel tanks
If we use 2 US gallons for taxi and 10 US gallons for the trip, what will be our landing weight?

Fuel Capacity

Total Fuel Quantity:  24.5 US gal. (93.0 liters)
Usable Fuel: 24.0 US gal. (91.0 liters)
Unusable Fuel: 0.5 US gal. (2.0 liters)

Its a bit unclear what you are asking. If 538 KG is your empty weight and 150 KG is your pax weight we would need to know how much fuel the plane holds to get the total weight. What ever the total is you will burn ~32.7 KG's of fuel and can use that for the subtraction. We would need a bit more info  to help out.

Fuel Capacity
Total Fuel Quantity: : 24.5 US gal. (93.0 liters)
Usable Fuel: : 24.0 US gal. (91.0 liters)
Unusable Fuel: : 0.5 US gal. (2.0 liters)

There isn't enough information to calculate the balance, but the weight can be calculated (from your original title)

First calculate your weight on the ramp:
$$
\mathrm{Empty~Weight} + \mathrm{Crew} + \mathrm{Fuel} = \mathrm{Ramp~Weight}\\
538\, \mathrm{kg} + 150\, \mathrm{kg} + 93\, \mathrm{L} \cdot \Bigg[\frac{0.72\, \mathrm{kg}}{\mathrm{L}}\Bigg] = 754.96\, \mathrm{kg}
$$
During Taxi, you said 2 gallons of fuel will be used:
$$
\mathrm{Ramp~Weight} - \mathrm{Taxi~Fuel} = \mathrm{Takeoff~Weight}\\
754.96\, \mathrm{kg} - 2\, \mathrm{gal} \cdot \Bigg[\frac{2.72\, \mathrm{kg}}{\mathrm{gal}}\Bigg] = 749.52\, \mathrm{kg}
$$
Then subtract the weight of the fuel that will be used on the trip:
$$
\mathrm{Takeoff~Weight} - \mathrm{Trip~Fuel} = \mathrm{Landing~Weight}\\
749.52\, \mathrm{kg} - 10\, \mathrm{gal} \cdot \Bigg[\frac{2.72\, \mathrm{kg}}{\mathrm{gal}}\Bigg]= 722.32\, \mathrm{kg}$$
(Source of conversions)


text:
Would one fighter pilot know that another's mask is not on?
I admit, this comes from the original “Independence Day” movie from 1996, but during the first dogfight scene one of the characters claims they can't breathe and takes their mask off.  Another character tries to order him to put the mask back on.  I assume that he couldn't just look into the other person's cockpit.  Is it possible in such a scenario to know that another pilot isn't wearing their mask?

Just a guess. Isn’t the radio microphone located in the mask. Except for helicopter pilot helmets, I have not seen any military pilot helmets with microphone booms attached. In a loud environment like a cockpit, it is hard to hear the pilot unless the microphone is almost touching their lips.

Not that I should be using Independence Day as the height of realism, but in the scene before the characters are seen talking without the masks on

The cockpits are pressurized, so they aren't on continuous oxygen.  The mask is on mostly in case of depressurization, and also because the mic is in the mask.  If a pilot is below 10000 ft and undoes his/her mask, they will have to hold it to their face when talking.  If the mic is 6 inches away because the mask is hanging down, the voice won't come through very well over the ambient noise.  You will know immediately if someone doesn't have his mask on as soon as they talk.

"...one of the characters claims they can't breath and takes their mask off."  Might this be how the other character knows?!

@MichaelStachowsky - I know what you mean. I was highly disappointed when my Delorean could not break the time barrier. . But seriously, almost nothing in Hollywood is done without some serious artistic license. Even physics takes a vacation in Hollywood. Especially when aviation is involved. Airplane! the movie could crack the top ten of the most realistic aviation depictions.

@MichaelHall Well, he claims he can't breathe, but he doesn't say "I'm taking my mask off" or anything of the sort

@John K, that would make a good answer.  I would just stress that pressurized or not, policy is for masks to be on from takeoff to landing.  Unofficially a rough rule of thumb is:  Auto pilot on, mask off.  Back to hand flying, mask on.

@DeanF. yeah my favourite is the radial engine sound effect from start to finish.

No, there's no mechanism to check if a pilot has their mask on. It's sometimes possible to hear the difference in how muffled the sound is. Other than that, it might just be that that particular pilot has a reputation for taking his mask off when he's not supposed to, and the other pilot just assumed that's what happened.


text:
Slotted wing loading and structural analysis
I'm trying to determine moments and forces on a wing with a fixed leading edge slot (trying to verify the Ragwing RW19 wing before starting the project). It's quite straightforward for a non-slotted wing, but when working with slots, I've run into several problems I don't know how to solve.

How do I determine the Cl and Cm of such wing? The XFOIL simulator is not able to work with two element profile AFAIK.
Should I calculate the wing loading for non-slotted wing only or should I also add the area of the slot?
Where is the aerodynamic center of the slotted profile? Is it around quarter chord (approximately) from the leading edge of the non-slotted wing or is it measured from the leading edge of the slot?


text:
Does filing a flight plan with Garmin Pilot activate ACAS and SE-SAR for my inReach?
I have my aircraft set up with a Garmin inReach for ACAS and SE-SAR on 1800wxbrief.com. If I file using the Garmin Pilot app, will the inReach ACAS and SE-SAR work or only if I file on the 1800wxbrief.com site?

If you don't get a good answer here, I suggest asking Garmin and/or Leidos. If you do, feel free to come back and answer your own question to help others, that's completely fine here.

SE-SAR and ACAS are only available for flight plans that are either filed directly through 1800wxbrief.com or through an app that's linked to your Leidos account (1800wxbrief.com > Account > Service Provider Authorization).
Garmin Pilot isn't on the list of known service providers, so FSS won't be able to match your flight plan with your online profile... meaning these services won't be activated.
Some apps (like Foreflight) have their own built-in ACAS feed, separate from 1800wxbreif.com's SMS-based interface. If your app has that, it would receive ACAS messages directly via push message. Unfortunately, Garmin Pilot currently doesn't have this feature.
See also this related question:
Will Lockheed Martin Flight Services send me ACAS, EasyActivate™, and other text messages if I file my flight plan through ForeFlight Mobile?


text:
How does Bernoulli's principle contribute in lifting airplane?
I have searched if any explanation that bind Bernoulli's principle in lifting airplane. There is one here which receive many appreciations, but still did not answer the question. So far, it was taught, explained, or mentioned in any aerodynamic or airplane principle, but never bound the Bernoulli's formula to the lift formula. As we know, there are pressure conservation as below:
$$\mathrm{P}_{1}+\frac{1}{2} \rho \mathrm{V}_{1}^{2}+\rho \mathrm{gh}_{1}=\mathrm{P}_{2}+\frac{1}{2} \rho \mathrm{V}_{2}^{2}+\rho \mathrm{gh}_{2}$$
Due to height is considered are same below and above the wing (the different are very small), then the third part of the equation will cancel one each other. Then left only part one and part two of each side of the equation. As the velocity above (considered as V1) is said will be different below the wing which wind is faster in above, then pressure above the wing will be lower, which will lift the wing. Thats is commonly taught in every explanation. Then, the
$$V_{2}=\sqrt{\frac{2\left(P_{1}-P_{2}\right)-\rho V_{1}^{2}}{\rho}}$$
Meanwhile, lifting formula is expressed as below:
$$L=\frac{1}{2} \rho v^{2} S C_{L}$$
V here is the airplane velocity, which is the wind hit the airfoil. Thence, V=V2 of the Bernoulli's equation above. V1 that hit upper side of the airfoils/wing, which is said faster than below, is unknown.
So, where is the Bernoulli's principle contributes in this case? How actually we calculate the lift force of an airplane?

https://aviation.stackexchange.com/q/21664/1467 https://aviation.stackexchange.com/q/8281/1467

That was in my mind since quite long. But you need to read comment from D  Sziroczak below. Interesting, as he said that the Bernoulli's factor is in the lift coefficient, which already in the Lift equation.

Related What amount of lift would be created in the fallacious “lift from equal transit time” theory?

1. Physical laws don't act, they only explain
The universe does not (seem to) calculate equation. It just works its way and we use equations to try to find some order in it that would allow us predict how it will react to what we plan to do.
2. Physical laws all hold at the same time
Physical laws don't describe parts of how the universe works that would be added one to another. Rather they each describe an aspect of how it works all the time. Therefore it is not so that one law (Bernoulli's principle) would contribute a bit of something (lift) and then another would contribute another bit. Rather it is their combination that tells us the phenomenon will occur.
After all, the laws are expressed by equations. Each law is one. But they usually have many free variables. One equation with many free variables constraints the solution, but you need as many equations as you have free variables to get a unique solution. Bernoulli's equation is not enough.
3. Applicability of Bernoulli's principle
Bernoulli's principle is just an expression of conservation of energy. We have all the reasons to believe that conservation of energy (of mass-energy) holds everywhere in universe, which includes around the wing generating lift.
But as said above, Bernoulli's equation is just one equation with way too many variables to produce solution on its own
4. Other laws
Due to the unbounded nature of the situation, the only way to get enough constraints to actually find a solution requires turning to the ultimate hammer of fluid dynamics, the Navier–Stokes equations. This is a set of complex equations that involves inertia and viscosity of air, both essential properties for generating lift. Inviscid flow does not produce any lift as can be tested in liquid helium. Massless flow wouldn't produce any either, but unfortunately there are no massless fluids to test it with.
You still have to throw in conservation of energy, i.e. Bernoulli's equation, along with conservation of mass (actually, they are both considered part of the Navier-Stokes set), and for larger pressure differences also the ideal gas equation and the equation for adiabatic process to get enough equations to restrict all the free variables.
The result is a set of partial differential equations that don't have any useful analytical solutions and need to be numerically integrated over sufficiently large volume of space surrounding the wing and sufficiently long time period.
Now you'll have huge dataset describing the flow at each point in space and time with some granularity. If you plot the total lift over sufficient set of boundary conditions, and try to fit a simple equation to it, you'll get the famous lift equation. Approximately—it is just fitting to a bunch of points!
5. “Contribution” of Bernoulli's principle
Bernoulli's principle does contribute to the explanation by holding in the situation. You wouldn't have enough constraints to get a unique solution otherwise. But there is no way to specify what the contribution meant in the resulting equation. All you can say is that it is needed to calculate the exact points that can be approximated with the lift equation.
Note: there are decent qualitative explanations of the phenomena described by the Navier-Stokes equations, but you've already seen them as it is the answer you linked in the question. No point in repeating them here.

"inviscid flow does not produce any lift" perhaps an apter way of saying that is "inviscid flow cannot get any circulation?"

@MeatballPrincess, yes, but that would mean introducing another term and what it has to do with lift.

If I understand correctly you want to know how to get from the first formula to the second one. If that is the case, the answer is simple: you don't :)
The lift formula is a simplified one, which considers the wing as a whole. It doesn't use velocities on different points of the wing, it uses the speed of the airplane. All the factors such as profile, wing shape, etc are included in the lift coefficient. This formula is useful when you already know the behavior of the wing in different situations (the behavior is determined in wind-tunnel tests or with numerical methods).

Yes, you got my point. So the question is, where is the bernoulli's principle in lifting the airplane? Is bernoulli's contributing in that thing?

@AirCraftLover No, not really. Bernoulli's principle is not a correct explanation for the creation of lift (https://www.grc.nasa.gov/www/k-12/airplane/wrong3.html).

Bernoulli's principle itself is perfectly correct. Out of all of the explanations of lift that mention Bernoulli's principle, some of them are correct and some of them are incorrect.

The statement "Because of continuity, so the air that flows in from the left side must flow out at the right, the upper flow must get there at the same time. But because that line is curved, air has to go faster to get there at the same time." is not necessarily correct.
See Chapter 3.2 from this e-book, excerpted here. https://www.av8n.com/how/
The author simulated a wind tunnel, and documented air flow patterns around a wing, with different colored chunks of air in different colors. He demonstrates that air above and below the wing do not arrive at the same time. The book also discusses quite well in my opinion how downwards airflow at the back of the wing really contributes to the lift created, more so than any low pressure area above the wing. Air being pushed down = weight of aircraft being pushed up. (The old "opposite and equal reaction".)

So, can I say that there is no Bernoulli's countribution in lifting process? I agree with you, the air is forced downward by the curved airfoil will act as agaist gravity (mg).

I don't know about No contribution, but I would suggest perhaps not the main player anyway. See chapter 3.6 of the same e-book. https://www.av8n.com/how/htm/airfoils.html#sec-airplane-air

Probably that the best way to conclude, that not the main part.

downwards airflow at the back of the wing really contributes to the lift created Not that it doesn't contribute, but most of the lift is created at the front of the wing. The lion's share of the downward airflow at the back of the wing was turned downward by the front of the wing.

Then the question is, "why is the airflow turned downward?" It's turned downward because of the low pressure above the wing. Both are happening simultaneously

"So, can I say that there is no Bernoulli's countribution in lifting process?" - No, you cannot say that. Bernoulli's principle accounts for 100% of the lift (ignoring viscosity).

See, here is the conundrum of introductory aerodynamics/fluid mechanics books. Lift is hard. There is simply no simple way of explaining lift. Why would there be? It is only fair that you need quite some math to figure out the pressure distribution, which is the pressure field around an arbitrary body in airflow, and as you can imagine, that is no easy task. Who gets to say that just because lift is essential to flight, it must be readily understandable?

First off $V_{\mathrm{free stream}}\ne V_1\ne V_2$, and $P_{\mathrm{atmosphere}}\ne P_1\ne P_2$, so here is your answer.
Secondly in a more accurate analysis $V_1$, $V_2$, $P_1$, $P_2$ can not be assumed constant at either under or over the wing.
A more accurate portrayal of lift is most easily achieved by simplifying the airflow first to 2D-potential flow, i.e. $\exists \varphi, v=\nabla\varphi$ then assuming $\rho=\text{const}$, then we would get $\nabla^2\varphi=0$ the Laplace equation. The Bernoulli's equation is used here to link $p$ singlesidedly to $v$ hence $\varphi$, if you apply Bernoulli's equation right from the beginning, how do you even know $V_1>V_2$ without making it an assertion?
(A ramification of making the flow a potential flow is we automatically set $T=\text{const}$ as well.)
Now we have only a single variable, namely $v$, and there are multiple interesting ways to solve the Laplace equation. But perhaps the way that provides the most insight into lift generation is through a conformal mapping, i.e. $f\colon\mathbb{C}\mapsto\mathbb{C}$ analytical and $f^\prime\ne0$. Conformal $f$ has the property that if $\nabla^2\varphi=0$ then $\nabla^2(\varphi\circ f)=0$ and $\nabla^2(\varphi\circ f^{-1})=0$.
As you can see where this is going, we study the potential flow $\varphi_0$ around a cylinder then find a $f$ that maps the cylinder to the wing profile and $\varphi=\varphi_0\circ f$ automatically yields the potential flow around the wing.
There are three kinds of basic solution to the flow around the cylinder: rectilinear, vortex and doublet. And per the linearity of the Laplace equation, any superposition of the three solutions is also a solution. Similarly, any flow around a wing can be seen as superpositioning of three respective $\varphi_0\circ f$.
Below is the visualization of the solution of the pressure field of flow around a cylinder (red, purple = high pressure, green, blue = low pressure, free stream flows from right to left):
.
Note that just like I said, there are three basic solutions, the pressure shown above is the result of changing the coefficient of combining the solutions.
Here is the airfoil version, note the high degree of similarity
.
A most useful result of this approach is a direct proof of Kutta–Joukowski theorem. This theorem states that the lift, defined as the component of net force acting on a body immersed in rectilinear airflow that is perpendicular to the free stream's velocity vector, of an arbitrarily shaped body in an inviscid potential flow is given by
$$L=\rho V_{\infty}\Gamma,$$ where $\Gamma=\int_C v\,\mathrm{d}s$ for any route $C$ that surrounds the body. This tool, not the Bernoulli's principle, is the real workhorse of aerodynamicists.

Speaking of potential flow and Bernoulli's equation, here is an interesting fact:
From the differential form of the momentum equation
$$\rho\dfrac{\mathrm{D}}{\mathrm{D}t}v+\nabla p=0$$
with the assumption that $\rho=\text{const}$, $\dfrac{\partial(\cdot)}{\partial t}=0$ (steady flow) we get
$$\rho\dfrac{\mathrm{D}}{\mathrm{D}t}v+\nabla p=\rho(v\cdot\nabla)v+\nabla p=\nabla\left(\rho\dfrac{v\cdot v}{2}+p\right)=0,$$
which suggests the total head $$H=\rho\dfrac{V^2}{2}+P$$ is constant not only on streamline, but everywhere in the whole domain! This is a stronger version of Bernoulli's Law, implicit in the Newton's second law.

Please do notice I have only mentioned lift under the ideal circumstance of inviscid potential flow, and the solution given by this theory deviates from real life in a significant way. For example, you can tell from experience that there is no way a cylinder can stand in water flow and not feel any drag, yet the solution to flow around cylinder says so. This is called d'alembert paradox. The answer to this paradox is viscosity of water. The viscosity of water prevents a full pressure recovery on the rear half of the cylinder, and the flow would separate near the top and bottom of the cylinder. Viscosity is also important for airfoil, firstly because it is the primary reason why wings stall, secondly, it has an intricate relation to exactly which solution will be established around the wing, and this determines through K-L law the lift, i.e. frictional drag actually dictates pressure drag and lift!!!

EDIT: The lift equation is $L=\frac{1}{2}\rho C_LAv^2$. As you would have guessed it, the circulation $\Gamma$ is proportional to airspeed. But the reason why increasing airspeed increases circulation is even more arcane and too long for one answer.
EDIT: The lift equation is derived from the K-L law mentioned above. $C_L$ is defined as $\frac{L}{1/2\rho v^2 A}$, not obtained theoretically from the airfoil shape and $A$, as the equation would have you erroneously believe.

Easy to illustrate. Hold a piece of paper loosely by the corners so it is curved down hanging away from your face. Blow gently across the top of it and see it lift. There is no "push from below", only a suction from above.
As a 700+ hour pilot I enjoy watching my wings get 'sucked' into the wild blue yonder by the lower pressure above the wings and the relatively higher pressure below - also known as 'Lift'

As I have explained in the question body with the two formulas, there is no equation which bind the bernoulli to lifting force.

The Bernoulli equation is defined between 2 points in a flow field. It has limitations, mainly that it's valid only for incompressible flow, so only valid for low airspeeds. As you noted accurately, the height term is usually ignored, but it's more to do with the low density of air than the size, as for water or mercury it would make a large difference even with small lengths.
So the equation you derived (apart from rho being at the wrong place, it should be p/rho)  is accurate between any 2 points in the flow field. Lift is caused by the pressure difference between the two sides of the airfoil, so if you wanted to calculate the lift, you would need to evaluate pressures at all points on the upper and lower side of the airfoil. As the Bernoulli equation states, where the flow speeds up, the pressure drops, and this is the upper side of the airfoil. See picture from wikipedia, the bottom streamline is almost straight, so its shorter than the curved one on the top. Because of continuity, so the air that flows in from the left side must flow out at the right, the upper flow must get there at the same time. But because that line is curved, air has to go faster to get there at the same time. Faster flow = lower pressure.

So you can use the Bernoulli principle to estimate the pressure at every point, provided you know the speed at the given point. Once you do that, you get pressure distributions over the airfoil, something like this:

Note that these distributions are not calculated by hand, but rather with some CFD code, or measured in wind tunnels. Once you have the distribution, it can be summed to get the total forces. To make life easier, engineers measured all these forces as a function of altitude, speed (Re number) and angle of attack, and made it available for you as a CL-alpha curve. Then the lift equation is a similarity law essentially, where you turn the non-dimensional pressure distribution, eg the CL into actual force that you can use to calculate.

@D Sziroczak, sorry for the wrong equation. Hope now it is correct.

Dear Sziroczak, I quote this: "the air that flows in from the left side must flow out at the right, the upper flow must get there at the same time. But because that line is curved, air has to go faster to get there at the same time". I heard that one very often. But there is no evident confirming that claim. Also NASA in their website still wrote that the Bernoulli's contribution in lift is still in debate.

From this statement: To make life easier, engineers measured all these forces as a function of altitude, speed (Re number) and angle of attack, and made it available for you as a CL-alpha curve. Then the lift equation is a similarity law essentially, where you turn the non-dimensional pressure distribution, eg the CL into actual force that you can use to calculate. Can I say that by using coefficient of lift in that lift formula as I wrote above, I have include the Bernoulli's contribution in that lifting force? Please, I need your feedback, Sir.

Because of continuity, so the air that flows in from the left side must flow out at the right, the upper flow must get there at the same time.  What says it has to get there at the same time? It doesn't. In fact the upper flow generally gets there before the lower flow.

AirCraft Lover: The Bernoulli equation is a way to describe properties at 2 points in the flow space. It does not "contribute" to anything. The lift is generated because of the pressure differences. Individual pressure differences can be calculated using the Bernoulli equation (keeping in mind the limitations). Lift coefficient is a simplification based on similarity law, so you don't need to calculate pressure distribution all the time, but assume that under similar conditions, pressure distributions are similar.

TomMcW if you have the undisturbed flow on the left, then you have a mass flow that's flowing in. By the time it reaches the far field on the right = undisturbed flow field again, the whole flow needs to be at the same speed (save for the effect of the downwash) If some parts of the flow are moving faster then they will slow down and speed up other parts of the flow through viscosity. And the total mass flow out will be exactly what flows in. Law of continuity must apply if you look at the whole flowfield. What flows in must flow out. If not you are creating/destroying matter.

I suspect this is what vortexes are all about.  No we are not creating/destroying matter.  We are converting thrust into lift and drag.  Do not forget the system energeticly is NOT in balance, power from the aircraft must be constantly added.  Air deflection is a CONSEQUENCE, not a CAUSE, of the airfoil creating lift.  Lift may be more Coanda than Bernoulli.

Robert, there are 2 principles in any fluid mechanics calculation, conservation of momentum and conservation of mass (+conservation of energy if those effects are significant, in this case not really). Conservation of mass means, whatever flows in must flow out or if its a closed system, then no mass is created or lost during the process. Conservation of momentum means that when the lift is generated (force acting up), then the airflow is deflected down (the reaction on the flow). Any theory you come up with must satisfy these two.

Please see this smoke test video: https://youtu.be/6UlsArvbTeo The air above the airfoil reaches the trailing edge before the air below and at a higher velocity. What you’re stating here is the “equal transit theory” which is incorrect. See NASA website for why it is incorrect.

You started well and I was about to upvote your answer, but then you fell for the equal transit fallacy. Please accept that continuity is still possible even when the air on the upper side transits the wing faster than that on the lower side. It is only about how much flows in on one side and how much flows out on the other side. How long the air remains within your control volume is of no consequence.


text:
Is repeatedly pitching an aircraft up and down to help it lose its energy quicker a viable landing technique?
Say a pilot "bobs" his aircraft's nose up and down. The rapidly changing AOA should generate more drag than if the aircraft were to fly straight at a constant pitch where the AOA shouldn't be significantly large, right? Is the extra drag significant enough for this technique to be used in a practical landing approach?

First it won't be very effective unless you stall it. Secondly it's not comfortable both to the human being and to the aircraft, so you scare your passenger and wear out your equipment. Also unless you stall, you will be climbing at high AoA, so your normal descending would become multiple cycles of diving and climbing. That less efficient but to effectively lose energy you need to be "horribly inefficient", things like air brakes, spoilers.

While, this would almost certainly have some effect, there are better, safer, proven techniques to control an aircraft's energy. A slip is the best I can think of regarding technique.

The idea of a good landing is not to waste energy, but to approach with the least energy possible (continuous descent), which is actually gliding. This is why low cost airlines start the descent earlier, to glide longer.

@user3528438: Or just a slip.

Are you presuming he has excess energy early on approach that he needs to dump, and will be squared up and lined up prior to short final?  Are you basically looking for an alternative to slewing as an energy dump?

@mins Is continuous descent really gliding?  How do you shoot an ILS which IIRC has a 3 degree glide path?  I'm not being snarky, it's not something I'm familiar with

@DaveGremlin: The goal of the continuous descent until the GS interception is to glide it, and then use engines. There is also a non precision "continuous descent final approach" which is the regular powered-flight glidepath from the FAF to the threshold without leveling at the MDH. In both cases, the final segment is powered as you mentioned.

@mins: Also, the glideslope and such is fairly irrelevant to those of us who fly VFR, mostly in single-engine prop planes going into non-major airports.  Like one I used to fly from, that was tucked in against a mountain range, with a gravel pit uphill from the usual approach end...

Well, it definitely works in Kerbal Space Program...

No, that kind of landing technique is too risky and dangerous.
You’re most likely to just exchange kinetic energy for potential energy, gain altitude, then slam into the ground with the aircraft exhausted of airspeed.
Also it’s superfluous, considering that a properly managed approach on glide path and airspeed will accomplish pretty much what you intended to do with your proposed landing technique.  A stabilized approach and entry into the roundout will result in the aircraft exhausted of energy and settling onto the runway once the power is pulled to idle, with little float.

The constant changes in vertical and horizontal speed would make it almost impossible to assess whether you were maintaining a reasonable landing profile. This might possibly make sense in an emergency where you had no other choice, but otherwise, why not just start the landing over without the excess energy? Simple, safe, and effective.

@David Schwartz: Because sometimes you need to descend steeply to avoid obstacles &c.  There are also places where go-arounds aren't really possible.

and beside being dangerous it is also useless: drag goes with speed squared, so just go fast and get is a sideslip. It is very effective.

Landing is pretty tricky:

it can only be done in certain areas with a suitable landing strip;
contact point with the ground should be at the start of the runway;
vertical speed should be kept to a minimum;
the speed vector must be aligned with the runway;
lower horizontal speed is convenient for correcting wind gusts etc.

So the aircraft speed must be low, while the speed vector must remain carefully pointed at a specific point on earth. Both these requirements are equally important for a successful landing, and voluntarily varying one for improving the other is not a good idea. Which is what you would do if you would be pumping the stick forward & aft.
In order to reduce airspeed, the drag of the aeroplane must be increased - this can be done significantly by increasing Angle of Attack or sideslip, where AoA variations also change direction of the speed vector. The change in pitch attitude (a rotational velocity) contributes very little to drag: rotation axis is the wing centreline, so only an extra speed around the horizontal tail surface is gained. Nothing compared to what happens with the wing and fuselage surfaces.
Repeatedly stalling and recovering the aircraft could be a way to quickly lose energy. Still at the cost of the vertical speed which will not be constant.
As @Ryan Mortensen points out in a comment, a much more controlled way of losing airspeed is by slipping: the sideslip angle increases drag while leaving the lift vector alone. Much easier to hit that suitable spot on the landing strip.

In R/C model aviation beginners sometimes do inadvertently try this technique. While they are on the final they keep working the elevator up and down up and down because they can't really judge a good glide slope at that stage of their learning cycle.
So when they do this on approach, it also happens after touch down. You have a bumpy ride going down to the landing strip you are going to have a bumpy landing and more often than not a broken prop and a nose wheel.
Why does it happen? Because the very basic premise is incorrect. Final approach is a very stable glide slope. If you try anything fancy there you can’t expect a great landing.
Also, when you are coming in like that and touch down you will have a natural tendency to overcorrect the pitch on the first bump and it then becomes a catch 22

Yes.  Got most of the cost of a major overhaul covered by insurance because one of my partners did that very thing on landing: extreme porposing, prop strike, required teardown of engine.

You can do side slip if you want to loose energy which is much more easy to do and probably more effective. Also fighter pilots can bank hard if the want to loose excessive speed on base turning final.

No.  This would be the direct opposite of a stabilized approach, moreso it would be opening  the possibility to cause an accelerated stall.
As has been mentioned, it would be an affront to the aircraft and to the humans aboard this aircraft.
Trying this is a sure sign of a bad approach, and would be better replaced with a go around and a better controlled second attempt.

The issue you face here is that pitching will affect your rate of descent rather strongly, which is exactly what you don't want while you're trying to land. Keeping on your glideslope is already tricky enough at times; trying to follow the glideslope while deliberately wavering up and down would be very difficult indeed - and when you start getting close to the ground, obviously the pitching risks bumping into the ground earlier than you planned, and at a flatter angle, which risks hitting the ground with something other than your landing gear (depending on your aircraft's configuration and how sharply you pitch down).
On top of that, landing brings you close to stall speed, and rapid changes to your AoA will therefore risk stalling the aircraft, and given that you're getting quite close to the ground during landing, you may not have the altitude to spare when recovering from that stall, which would result in a hard landing or crash somewhere short of the runway.
The principle, however, is sound. Repetitive use of maneuvers that increase drag in order to reduce the aircraft's speed is the idea behind "fishtailing", which is basically the horizontal equivalent of what you describe. Basically, a fishtail maneuver involves pushing the rudder over to one side or other while keeping the wings level, resulting in a slip. Ordinarily you'd push it to one side for a moment, then let it come back and do the same thing the other way, resulting in a motion not unlike a fish swimming, hence the name. This reduces your speed whilst having very little impact on your overall rate of descent (aside from the effects of your reduced speed), making it easier to achieve safely.
All the same, I probably wouldn't advise fishtailing as a deliberate intention unless you absolutely must land first try, for whatever reason. If you feel you're coming in too fast and you're not sure you can slow it down in time, you're probably better off going around and having another try at getting it right.


text:
How do pilots rest on long-haul flights?
When flying on a long-haul airliner flight in economy, often I find it very hard to fall asleep. However, when off cockpit duties, they still have to get rest so that they are able to control the aircraft without being exhausted and collapsing on the controls when they are on duty. Are there are any specific methods of helping pilots to gain the rest required on board an aircraft? Have there been any studies about the effectiveness of these?

Unlike passengers in economy who are trying to sleep in sitting position in seat that can be only partially reclined, the pilots sleep lying in a bed. Long range aircraft have a crew rest area. They are often above or below the main cabin and therefore low, not allowing standing up, but they always have beds or fully convertible chairs. The flight crew rest area is most commonly for 3 and separate cabin crew rest area for whatever is the usual number of flight attendants in given plane.

This also goes by the name of crew rest compartment(s) (google reveals some interesting images).

Since a picture speaks for a thousand words...
These are often called "coffins" or "sarcophagus" around here... (Portugal)

Source

Ahhhh... blue light makes everything better :D

That photo shows a B777 crew rest area for flight attendants, not pilots.  Here is a link to the pilot's crew rest area: http://www.airliners.net/photo/Boeing/Boeing-777-240-LR/1061577/&sid=781090153121e4955b239df2b638883a


text:
how could engineers find pressure loss location in jet engines?
What are the tools, software or devices that jet engineers use to know if there is loss of pressure in the air? Are they using only calculations to know how much air in vs out?
It's very interesting to know how they could find pressure loss when they design their jet engine exactly in order to reduce it?

What do you mean exactly with pressure loss?

for example, when the fan sucks the air or the compressor, what I know is there is some loss of compressed air, so stator blades angle help handle that to avoid losing much pressure. so, I WANT to know how they measure pressure and see if there is loss of pressure once enters the compressor and get compressed

"stator blades angle help handle that to avoid losing much pressure" actually the stator blades do create the pressure losses you are talking about but they are anyway necessary to guide the flow towards the next compressor stage. Some other pressure/energy is lost due to the not perfectly sealed interface between blades and outer wall. Is this what you mean?

Pressure loss is something you encounter due to the shape of the inlet, fundamental pressure loss of heating air, etc. So designing for minimal pressure loss is designing a machine using the laws of aerodynamics. Gas turbines are designed to have minimal pressure loss, encountering a sudden pressure loss in the air is just a different operating condition. Your question is not clear. Please note, shouting (using capitals) is usually frowned upon, please improve your question.

Pressure loss is not the only and also not the most important variable governing engine performance. In general, engineers try to compare calculated performance number such as pressure, temperature, flow velocities, mechanical stress and other parameters to calculated metrics. This way, the performance of the engine can be validated and compared to the expected performance which was calculated during the design phase of the engine. For these tests, a couple of engines are instrumented with suitable test equipment which shall measure some of these numbers at critical locations throughout the engine.
I tried to summarize some measurement techniques in the following list (which is by no means comprehensive):

Pressure and temperature probes: Simple pressure or temperature probes which can be inserted into the casing at various points

Velocity probes throughout the flow section: Pressure transducer inserted into the casing to give spatial distribution of flow. See the following picture

Strain gauges: These instruments measure the lengthening of a blade for example and can be used to calculate mechanical stress

Optical beams: These devices are inserted such that their beam is aiming at the rotating blades of the engine. This technique can be used to gather additional information about the flow around the rotor blades

Please note that I took all of the images out of MTUs Measurement techniques and instrumentation  company leaflet. Other resources can be found for example here or here. Especially the first source gives a good thorough overview.
I hope this gives a very rough overview.


text:
How did this Vampire's takeoff damage the runway so much?
This appears to be real, but, what's the cause?
Aircraft is a DeHavilland Vampire on takeoff from a small airfield, but the runway's surface disintegrates from jet blast
Full file at https://i.imgur.com/NaK5yAr.mp4

Is the Vampire's jet blast that strong?  Or is it weak/old tarmac which is being pushed over the limit of adhesion?
I suspect the field would be closed for repairs becuase of this damage, and the the runway's now lack of smoothness,  but could that plane land again right away?   If the loose bits were picked up?  Would it have to land on the grass verge instead?
Would this have happened with other aircraft on the same runway?  Or is the vampire special?

Other info
Plane's registration number is WZ507
https://www.facebook.com/Vampire-WZ507-351376189100/
This airframe is also the poster photo on https://upload.wikimedia.org/wikipedia/commons/thumb/d/d5/Vampire_3_%28cropped%29_copy.jpg/450px-Vampire_3_%28cropped%29_copy.jpg

FWIW the AAIB reported no such incident on that day, 2017 April 30.  Maybe because the aircraft itself was undamaged.

Partly it's that the Vamp's tail pipe is only a foot above the surface, angled toward it, so with most jets that wouldn't happen quite as easily.  Plus it looks to me like the airport operator cut corners with a really cheap driveway quality paving job with an asphalt layer only an inch or two thick.


text:
Can I preheat my C150 by placing a space heater in the cabin?
I have a C150 in a hangar, but no jet heater.  I was planning on just putting a directional space heater (an FH-06B) in the footwell and opening the cabin heat to let the heat through to the engine.

I figured that running that for an hour or two would be better than 20-min with a jet heater and almost as good as a heated hangar.
Testing this out, the rudder pedals get very hot, but not so hot that you can't touch them.
Are there any flaws with this plan?

It'd be more effective to place the heater in front of one of the air intakes in the cowling. As it is now, you're mostly heating the cabin instead of the engine.

Closely related

Almost no heat gets to the engine.  What hot air drifts through the heater SCAT hose  might warm the exhaust muff a bit.  Plus you have that fire hazard heater in the cabin.
Get some of that aluminum flexible ducting used for clothes dryers and form the end to fit over the heater, and tape it on with aluminum speed tape. Set the heater on the ground to the side with the duct stuck in the bottom of the cowl.  Put some foam in the cowl inlets. 30 minutes to an hour is all you need.
A lot of people just stick the heater itself under the cowl but it's way safer to duct the air in with the heater outside and works just as well.
Preheating is traditionally all about warming up thick oil and the galleries the oil has to flow through, not so much heating the metal itself for the sake of the metal (the cylinder doesn't much care if it warms to 350F starting out at 20F or 50F as long as oil is getting to the parts).
If the OAT is above 20F and you are using multi-grade oil like Aeroshell 5W50, the oil will flow fine at ambient temperature and there is really no need to preheat if the rise in oil pressure is not significantly delayed from warm weather starting. Not saying not to do it, but if I had no preheater and it was 20 degrees, and I was using multigrade oil, I wouldn't ground myself and would start the engine anyway.

This won't work well as there's no way for the hot air to get from the cabin to the engine directly. The airplane is designed to prevent air getting from the engine bay to the cabin, and vice-versa. Cabin air is heated by exchanging heat with the engine exhaust system, the air is forced through that duct from the outside airflow over the cowling. So there will be nothing to force air back the other way. What heat does go that direction will heat the engine exhaust system a bit, however very little is going to reach the engine.
Heating the cabin will heat the whole airplane a bit as the heat conducts through the metal, but it's very inefficient and unlikely to be enough for your purposes.


text:
How do you use a flexible drive shaft?
Saw it on a 747 freighter nose door, see it again on an F16's M61A1 gun assembly.  How do you design with a flexible shaft? In what situation do you use it to replace a bunch of rigid shafts and gears? Does it provide the strict angular synchronization of normal rigid driveshafts? Or is the synchronization only approximate, and can only be used in situations like either the 747 nose door or M61A1 gun-to-drum drive where there is only a requirement for a rough 1-1 translation of motion and power and no real demand to perfectly synchronize? Can you guarantee that if there is a constant speed revolution in one side you would get a constant speed revolution in another?

It's not really clear what you're asking. Mind to give an example?

Flexible drive shafts are also used in engine, e.g. for VBV system in a CFM56 (see diagram in the answer of this question)

Flexshafts are used on a lot of flap systems to interconnect an array of screwjacks.  They are more or less aerospace versions of weedeater shafts, inside a hydraulic line style casing (typically stainless braid over a teflon liner).  They are used because the envelope and clearance requirements are much easier to meet than with solid shafts with angle drives and gearboxes.  You can route them through very tight spaces and around gentle corners without having to maintain clearances to adjacent parts; you just have to keep the flexshaft housings from chafing on adjacent parts, like hydraulic lines.
Flexshafts aren't meant to take high torque like solid drive shafts and are normally used in high RPM low torque applications.  Or better to say that if you want to use a flexshaft, you design your drive system to run at high speed/low torque.  A drive line might be designed to run at 500-1000 rpm with solid shafts and a moderate gear reduction at the actuators, whereas a flexshaft will run at 2 or 3000 rpm at much less torque, but with a much higher gear reduction in the actuator to get the same output force result.
Flap system flexshafts as used on CRJ200s and Embraer 145s will only have to sustain a little less than 100 inch pounds of torque at operating load, while producing several thousand pounds of force at the end of each screw jack thanks to the gear reductions in the actuators.  You get some torsional displacement, or wind-up twist, from one end of a drive line to the other, maybe 1/3rd of a turn, but with the high speed and large number of revolutions (say 800 to 1000 turns) to cover an operating range, a third of a turn of wind-up doesn't have a major effect on position sensing, so synchronization isn't a problem.
It'd be the same deal on a 747 nose door.  The flexshaft connecting the door actuators runs at a high enough speed so that synchronization down to a fraction of a turn is not really necessary because each turn is a very small part of the total motion.  Cascade thrust reversers on the CF-34 engine are also run by screwjacks interconnected by flexshafts; same thing there.
The big downside to flexshafts if used in exposed areas is sensitivity to contamination through imperfectly sealed connections.  Also the carbon steel wire windings tend to suffer from fatigue where they are crimped at the terminals.  There is a lot of back and forth flex as they spin.  They usually start to break down as individual strands fracture, and the bundle swells out and jams in the casing or sometimes lets go completely.

Flexible shafts do are just shafts and do not contain any gearing or otherwise change of angular rate. They may, depending on their construction, be subject to a load related skew (torsion) - like any 'rigid' shaft as well. Nothing in this universe is absolute inflexible.

on a M61A1 gun when the gun is rapidly spooling up. So the hydraulic drive is located on the gun, rather than on the drum, and the two are synced by a flexible shaft only. The gun demands a round in it first, and due to the flexibility, the drum will give a round after the shaft has taken up a certain amount of twist. If according to the other answer, the shaft always spins fast then 1/3 turn of a twist doesn't matter, as the feeding chute will be stretched a little to accommodate that. But if the flex is excessive say even for half a round's diameter, ~15mm, then the conveyer belt will break.


text:
How does propellers' pitch change during the use of reverse thrust in turboprop/propfan propelled planes?
Which way do proplellers turn during the use of reverse thrust in turboprop/propfan propelled planes:
A) Do the leading and trailing edges of propellers remain the same or...
B) Do the propellers turn in the opposite direction, so that the leading and trailing edges of propellers swap?
I tried to show what I mean in the picture:

source: https://es.123rf.com/photo_69655677_cerca-del-motor-de-turboh%C3%A9lice-del-avi%C3%B3n-con-h%C3%A9lice-partes-del-fuselaje-de-un-avi%C3%B3n-alas-y-cola-sobre-fondo-de.html

They are more like an aerobatic airplane that's flying upside down.  The wing of the propeller blade keeps moving in the same direction but it just rotates so it operates at a negative angle of attack to make lift in the opposite direction from normal.  The blade is in a socket in the hub that allows the blade to rotate on its long axis.  The blade root has a big gear on it and there is a gearset within the hub, driven hydraulically on most props using engine oil, that drives gears simultaneously to rotate the blades.
The only difference between a reversing prop and a non-reversing one is the range of rotation of the blade; the reversing one can rotate far enough that the front face of the blade becomes the "bottom" of the wing so to speak when in reverse.
The magic mostly happens in the control system.  In forward flight there is a flyweight governor (like on a steam engine) that controls blade angle based on an RPM setting from the cockpit. Blade angle is adjusted to keep the RPM set.  Reversible props have what is called BETA mode.  In BETA, the angle of the blades is controlled directly by the throttles (power levers) down through flat pitch (DISCING, that is, the prop is effectively a big disc) and into reverse.
You come into BETA when you come back to flight idle.  This is "flight BETA".  On the ground, after you land, you yank on a latch of some kind and move the power levers back more and the props go into ground BETA into DISC, then reverse pitch.  All this time, in BETA, the rpm is controlled by an engine speed governor that adjusts engine torque to set RPM.
BETA mode is more or less how turbine helicopters work, where you directly control rotor blade angle with collective, and a governor adjusts engine torque to keep the rpm constant for you.

I think my question was a bit misleading, so I've edited it. Nevertheless a very informative answer - I suppose that from your post I can say the hypothesis "A" is true.


text:
Why is the Antonov 225 (almost) always chocked at the third wheel?
Recently I was looking over pictures where the An-225 is parked (because that's a normal thing for a normal person to spend their time doing). I noticed that the chocks are almost always placed around the third wheel from the front in the undercarriage:

(Thanks to ratchet freak for the better picture).
Presumably that's because the centre of mass (COM) is closest to that wheel? But why would the undercarriage not be centered around the COM?
Bonus question: Very occasionally, I've seen the chocks placed around the entire undercarriage, in front of the first wheel and behind the last wheel. Why the occasional difference?

There's a closer pic on wikipedia: https://en.wikipedia.org/wiki/File:Antonov-225_main_landing_gear_2.jpg (under CC BY 2.5)

The center of mass of an empty aircraft is at a known point; once the aircraft has stuff on it, the center of mass becomes variable.  During loading & unloading, the center of mass can vary quite a bit -- beyond the range which it's kept within for flight.

Note that you got the front wheel carrying some weight too, so your CG is probably designed to be a bit aft of the central wheel of the main undercarriage.

+1 for "that's a normal thing for a normal person to spend their time doing"

It's a matter of procedure.  Chocking a center wheel provides the greatest defence in the unlikely case the chock is jumped.  Other wheels in the truck must also contend.  If an outside wheel is chocked and that chock is jumped away, all is lost.
No matter tipping or rocking the aircraft in extraordinary winds or loading, the center of a wheel truck is the recommended place to chock.  A chock should be snug against both sides of a single tire to prevent any sway.
Most chocks are paired together by a rope or chain which would prohibit chocking around a large truck of wheels as described in your bonus question.  What's more, chocking at either end of a truck allows more elasticity and movement between the chocks than a single wheel chocked snug.

With chocks in front of the lead wheel & behind the aft wheel, if the aircraft goes forward, every wheel is then dealing with the chock nearest the nose, and vice versa if rolling backwards.  Small wheel chocks are typically connected with a length of cord; once the chocks are large enough for wheels like this, carrying two at once becomes cumbersome & they aren't necessarily connected.  Not sure I'm persuaded here...

Then why not chock the center wheel instead of the third wheel?

The chocks weigh a lot.  So long the truck is chocked toward the middle and not the end, there's no reason except exercise to carry them three feet further.

I dunno... it seems pretty weird that the standard procedure would require to walk all the way to the third wheel (for the sake of safety) but not to the fourth wheel (because gawwwd these chocks are too heavy). Do you have a source for this?

It's also possible (now I'm just making this in my mind, so I got no reference for it) that the requirement is not to chock the first or last two wheels. Then, if I were the mechanics guy carrying the chocks, I would soon develop the habit of chocking the 3rd wheel.

@Sneftel, the 4th to 7th wheel (axle, rather) are steerable on An-225 (or possibly free-castor; I've seen skid marks from them after pushing the aircraft back - in fact, you can see them on the photo). It makes a little more sense to chock a 'rigid', non-steerable wheel. In reality, I think, it's either a checklist requirement or a habit.


text:
What is the function of the aft stator in CFM's open rotor engine design?
CFM revealed their design for a open rotor propfan engine with two stages of blades - one active set of blades rotating in the forward and an static sets of blades on the aft.

According to The Seattle Times the design of this engine is:

…with scimitar-shaped rotating blades, and behind that a circle of fixed blades, or stators, that straighten the air flow.

Why is it important to straighten the airflow? Does it help in reducing drag, and/or noise, and/or increase thrust and/or more?
Why are such a static guide blades not used in contemporary turboprops? What are it's limitations? This question asks the exact same question, and the answer points out how inefficient it is to add stators to propeller configuration. What has changed in the latest design by CFM?
How is it ensured that the efficiency gains from this design is greater than the mass and drag penalty of the stator blades? CFM intends to achieve a 20% reduction in fuel consumption and CO2 emissions compared to current engines. This would mean (presumably) the design would be considerably efficient than a stator-less configuration


text:
How is the same takeoff performance achieved by a specific engine type on aircraft of different sizes?
The entire B737Max family (B737Max7,8,9,10) uses CFM International LEAP-1B (Source) engines, despite the B737Max10 carrying up to 60 more people than Max8.
My question is how do all of the aircraft in the family achieve the same takeoff performance, despite such a remarkable difference in their size, with the same engines?

Do you have a source that says they will achieve the same takeoff performance? I assume you are talking about takeoff distance required?

There are several model variations of that type engine.
Refer to the Type Certificate Data Sheet (TCDS) for the Boeing 737.  You will find listed the specific model number numbers and thrust for each approved engine.
For example, here are the CFM engine models approved for specific model/serial numbers of the 737.  You will have to get the latest revision of the TCDS for new models, like the 737 MAX.  This excerpt from the 737 TCDS shows the model variety of one type engine (the CFM56-7B) as approved for certain 737s: